Preheaters

A preheater is not necessary to the operation of a Stirling engine. It is a desirable accessory for all but small orimitive enuines and it is virtually

The function of the preheater is 10 heat incoming air en route to the combustion chamber using thermal energy in the departing stream of hot combustion products. This saves fuel and also endows the engine with the advantageous characteristic of a "cool" exhaust. Exhaust-gas/inlet-air pre-heaters are sometimes also used on steam (Rankine cycle) and gas turbine (Bravton cycle) engines. When thus equipped they arc said to operate on a 'regenerative cycle'.

Therefore, complying with customary engineering usage, a Stirling engine equipped with a preheater may also be said to be operating on a regenerative cycle. Thus in the Stirling engine there are two regenerators to consider: (a) the interna! regenerator interposed between the cooler and the heater through which flows the working fluid at high pressure and (b) the external regenerator which is the exhaust-gas/inlet-air preheat heat exchanger operating at, substantially, atmospheric pressure.

To add yet further confusion the Stirling engine external regenerator, or the exhaust-gas/inlet-air preheater. may be a heat exchanger that is of the regenerative or alternatively of the recuperative type.

A regenerative heat exchanger consists of a porous matrix of finely-divided material through which flows alternate streams of hot and cold fluid. A recuperative heat exchanger is one where separate flow channels are provided for the hot and cold fluid streams and ihrough which, in most cases, the fluid flows continuously.

Both recuperative and regenerative preheaters have been applied to Stirling engines. Early Philips engines all used recuperative preheaters, one of which is shown in detail in Fig. 12.7. l"his led to the characteristic large, cylinder head enclosure that can be seen on the engine illustrated in Figs. 12.12 and 12.IS. Recuperative exchangers can be made that are highly effective from the aspect of heat transfer. They are limited in effectiveness by the surface area for heat transfer that can be designed into a given size of envelope, a given mass of metal, or manufactured to a given cost. Units of the high heat transfer effectiveness required in Stirling engines tend to be relatively large, heavy, and very expensive assemblies of crimped or pre-formed thin-wall stainless-steel plates with inlet and exhaust headers or manifolds.

The MAN/MWM accordion type air preheater shown in Fig. 14.-1 is a promising approach to a low-cost effective recuperative heat exchange!. The core of the exchanger is a single strip of thin-wall aluminium folded repeatedly and contained in a metal case so that exhaust gas flows on one side of the folded sheet and inlet air on ihe other.

Cleaning is a particular difficulty with recuperative heat exchangers of compact form with fine flow passages. Condensed combustion products or

The solar Stirling engine is progressively becoming a viable alternative to solar panels for its higher efficiency. Stirling engines might be the best way to harvest the power provided by the sun. This is an easy-to-understand explanation of how Stirling engines work, the different types, and why they are more efficient than steam engines.